Bad Manners and Brimstone

Honestly, I think my answer depends a lot on the circumstances and the behavior of the person to be warned/not warned.

I'm a big fan of the idea that people should reap what they sew.  If the person whose identifying information was easily found was acting like a jackass, then I wouldn't go out of my way to warn them.  If, however, they were being reasonable (even if their opinions on a topic were different from mine), then I probably would warn them.

I'd want to know but, as Twik said, it isn't easy to hide your tracks on the internet.  Which is why you shouldn't post anything anywhere that you would want to stay private.

I would probably warn someone if it seemed likely that other people would make the connection, and that they were making any efforts to hide it. If someone is using something that looks like a real name (something like "Mary Jane Olsen" rather than "MJ_in_Minnesota" or "I love cookies") I'll assume that either they're okay with people associating their real names with their posts, or they decided that "Mary Jane Olsen" would be less conspicuous than "Queen of the May." I've noticed over the years that people who post with odd-seeming names are often assumed to be using handles/pseudonyms, but things that look like ordinary names are taken at face value.

I also wouldn't assume that Person1 was being careful about Person2's identity; if you start blogging as This_is_an_alias but tell a hundred of your closest friends that that's your alias, one of them may offhandedly say here's a cool thing my friend Jay said" and link to the until-them pseudonymous blog. This happened to a friend of mine, ages ago; the person who posted the link hadn't thought that maybe there was a reason that her LiveJournal, unlike his, didn't have her real name on it anywhere. Oh, as a meta-level thing, it's possible that this post contains enough information for someone to guess who I might be, the way "I went to high school with $person" compared with my current location might be. However, that would take a bit of work, unless you're one of a few of my friends, in which case PM me if you want to identify yourself.

I am friends in FB with a woman at work.  She and her friends were arranging a get together and they were all posting their phone numbers on the post.  The post was originated by someone else who I wasn't friends with, but I was seeing all of their phone numbers because I was a "friend of a friend".  I did an FYI private message to my friend letting her know if I could see the post of HER friend, then so can all of the other "friends of friends" and everyone who read it now had her phone number.

Shortly after I mentioned that, she modified her post, and I noticed that almost all of the other phone numbers were soon gone.